This invention relates to sealant compositions. More particularly the invention relates to electrophoretic coatable sealant compositions which are capable of being hot applied to metal surfaces for adhesive and sealant purposes and have the added capability of accepting electrophoretic coatings.
Adhesive and sealant compositions are widely used in many commercial and industrial production processes. A multitude of such compositions have been developed over the years, many of which were designed for a special use. Adhesive compositions generally are used to bind two substrates together Sealant compositions generally are used to form a load-bearing elastic joint between two substrates. The sealants also exclude dirt, moisture and other materials from the joint and form a smooth juncture at the joint and in some applications must be paintable. Necessarily, there is some overlap in the functions of the adhesive and sealant compositions.
The automotive industry is a major user of both adhesive and sealant compositions. Automobiles are assembled from several structural components. The components are joined together in various fashions depending on the particular components and the degree of stress that will have to be endured. For certain assembly steps an adhesive composition applied as a liquid and subsequently hardened provides sufficient bonding strength. For example, metal assemblies of door panels, quarter panels, tailgates, and roofs use adhesive compositions. These same assemblies also use sealant compositions at a later stage in the assembly line. Still other automobile assemblies which are welded or bolted together use sealant compositions in their seams. The wheel house, shock tower, rocker panel, firewall, floor hem flange, floorplan, and trunk are a few examples of where sealants, but not adhesives, are used.
Typical automobile body assembly lines contain a separate body shop and paint shop areas where adhesives and sealants are individually used in each area respectively. Adhesive compositions applied in the body shop area are normally high strength epoxy or modified-epoxy adhesives which are capable of bonding to oily galvanized steel. In some cases vinyl plastisols are employed, however, these applications are normally limited to situations that do not require high bonding performance. In any case, these materials are applied at room temperature and later cured through exposure to heat. A current weakness of these materials is that in order to apply them, they must be of low enough viscosity to be mechanically pumped with adequate flow rates. Normally, because of their low viscosity these materials are easily displaced when exposed to liquid impingement by various cleaning solutions (washes) to which the assembly body parts are exposed.
Recently a new class of materials referred to as hot applied adhesives have been introduced in the automotive body shop. These are urethane or epoxy-modified urethanes and are solid or semi-solid at room temperature. These adhesives function by changing from a flowable fluid at an elevated application temperature to a bonding solid at use temperature. The hot applied adhesives have a number of characteristics which make them attractive for use in the body shop area. They are usually solvent free and thus no special fume collection hoods are needed. They have low viscosities when made fluid and thus flow readily to fill gaps and wet the substrates to be bonded. Most importantly the resins after application provide a high strength bond and cure to an even higher strength after passage through drying ovens at a later stage on the line. Additionally, the applied adhesive compositions have a rapid set time. They will not be displaced when exposed to liquid impingement by various washes to which the assembled body parts are subjected.
The assembled parts as they move from the body shop area are subjected to sealing, painting, and final oven curing in the paint shop area. Sealant compositions used in this area must also have a certain set of characteristics to be acceptable. Vinyl plastisol compositions have been found to have those needed characteristics. Basically, a vinyl plastisol is a finely divided polyvinyl chloride resin suspended in a plasticizer. The plastisols are liquids which are applied at room temperature to the substrate. The liquid is converted to a solid through exposure to heat In effect, the heat causes the suspended resin particles to be fused or dissolved in the plasticizer. A solid product results upon subsequent cooling.
Vinyl plastisols are excellent as sealants in the paint shop area of the body assembly line. They flow readily at room temperature to fill seams and body joints which need to be sealed. They adhere well to primed metal surfaces. They can be painted over without leaching or causing other cosmetic problems. Finally, they are durable enough to withstand normal weather and user exposure. Another important quality of the vinyl plastisols is that they are not expensive. The plastisols would ideally be used in all adhesive and sealing phases of the automotive assembly process if they had better metal bonding properties and could withstand the washes and primer applications experienced in the body shop area of the process. In this regard, Gerace et al, U.S. Pat. No. 4,900,771, assignee the same as the present, discloses thermally stable hot applied plastisol compositions that are especially useful in the assembly of metal automotive parts. The plastisol composition disclosed therein can be hot applied to automotive parts for adhesive and sealant purposes, and can subsequently withstand the successive steps of a washing, top coat paint application and oven bake.
The method of bonding and/or sealing using the composition disclosed in U.S. Pat. No. 4,900,771 involves hot applying the plastisol composition to joint areas of a metal assembly, followed by cooling the composition sufficiently to achieve sufficient bonding. If an automobile body or a part thereof is being sealed in this manner, the method may in some instances further involve applying a primer composition to the metal assembly, heating the metal assembly to cure the primer, and applying additional plastisol composition to joint areas to form a sealed smooth surface In any event, it will usually involve applying a paint composition to the metal assembly, and baking the metal assembly to cure the plastisol composition and the paint composition.
After a paint composition is applied to the metal assembly, there is an appearance difference between the paint-over-metal surface and the paint-over-sealant surface. This appearance difference will depend on the particular paint and sealant compositions used and may range in degree from very slight to extremely gross including major surface imperfections such as tack, run-off, discoloration, cracking, peeling, cratering, etc. One reason for this appearance difference is that the metal can accept the electrophoretic primer coating and the sealer cannot. This results in a paint difference over painted primed metal versus painted unprimed sealer.
In this regard, heretofore no sealant and/or adhesive has been able to successfully accept an electrophoretic primer coating in order to alleviate differences in appearance between paint-over metal and paint-over sealant surfaces on an automatic body. Moreover, another benefit of having a sealer or adhesive accept the electrophoretic primer better than the sealer is that the coating can form a barrier over the bond line and protect the bond line from weakening due to the exposure of environmental elements such as moisture, salt, oils, etc.
Therefore, in order to remove any possibility of paint appearance difference regardless of the particular paint composition and sealant composition employed, and to also enhance the bonding capabilities under environment exposures, it would be highly desirable to have a sealant which would allow a primer composition to successfully be applied on the sealant by electrophoretic deposition.
In this way, such a sealant would be electrophoretic coatable--i.e. able to accept primer coating and thus, eliminate the appearance difference between the paint-over-metal surface and the paint-over-sealant surface The paint surface would have a uniform appearance over the entire automobile body surface. However, traditional adhesives and sealants when placed on galvanized steel panels act as electrical insulators thus precluding successful electrophoretic deposition of the primer coating. In this regard, for other reasons some adhesives incorporate conductive fillers in large amounts ranging from at least 35% to as high as 75% by weight. While these adhesives and sealants may have some electrical conductivity with this filler composition, the vital properties and functions of such adhesives and sealants can be destroyed as a result of the inordinate high filler content. As a result, they have not been used as electrophoretic coatable sealant compositions, as far is known.
In an unrelated art, Japanese Patent No. 63-140609 discloses using an epoxy resin adhesive for producing an electric distributor case. The Japanese patent discloses a conductive material incorporated into an epoxy resin. The conductive materials used include aluminum, zinc and graphite flakes. The preferred formulation requires approximately 35% conductive material and about 50% epoxy resin. However, the use of conventional graphite flakes in addition to other conductive material in these amounts would not preserve the properties of a sealant composition which is to be used on an automobile body surface. In that regard, the formulation of Japanese Patent No. 63-140609 is directed toward forming an electric distributor case which is not required to accept electrophoretic deposition of a primer composition. While the Japanese patent may disclose the use of a conductive filler with an epoxy resin adhesive, there still remains a need in the art for a sealant composition which can readily accept a primer composition, and thereafter accept a paint composition to thereby eliminate differences in appearance between the paint-over-metal surface and the paint-over-sealant surface.
Accordingly, there remains a need for a sealant composition which may be applied in the body shop over oily metal, and is electrophoretic coatable, thus allowing a primer composition to be applied uniformally over the entire automobile body surface without thereafter causing any imperfections in appearance after painting and to further protect the sealer bond line from environmental factors. Further, there remains a need for such a sealant composition which does not require large amounts of conductive fillers so as to preserve the properties of the sealant.